1. Field of the Invention
The present invention relates to an electrophotographic copying process in which a photoconductive layer is electrostatically charged and exposed to an information-carrying original. The latent charge-image obtained on the photoconductive layer is developed by means of a developer liquid to obtain a visible toner-image. Excess developer liquid is removed by an element which contacts the photoconductor surface, and the toner-image is transferred by electrophoresis from the photoconductor onto an image-receiving material and is fixed thereon. The photoconductor is then cleaned and/or discharged.
2. Discussion of Related Art
German Offenlegungsschrift No. 3,018,241 discloses a method for removing excess developer liquid from a photoconductive surface on which an electrostatic charge-image has been developed. The developer is composed of a suspension of charged toner-particles in an insulating developer liquid. In the disclosed method, a drying element in the form of a squeegee-roller or absorbent roller is brought into contact with the photoconductive surface. This squeegee-roller or absorbent roller is maintained at a potential having a polarity which is identical to that of the charge on the charged toner particles. In addition, the relative motion between the photoconductive surface and the squeegee-roller or absorbent roller is controlled to be zero in the contact region. The cylindrical surface of the squeegee-roller or absorbent roller is composed of an elastomeric material exhibiting a Shore-A hardness of less than 45 and a resistance value of less than 10.sup.9 Ohm.cm. The photoconductive surface is located on a drum which runs counterclockwise past a metering roller or stripping roller which is capable of limiting the quantity of liquid remaining on the photoconductor after the development of the latent charge-image. This metering roller or stripping roller does not touch the developed charge-image, so that neither streaks nor distortions are produced. After passing the metering or stripping roller, a layer of developer liquid with a thickness of between 10 and 15 .mu.m remains in the photoconductor surface and the surface of the drum passes over the squeegeeroller or absorbent roller. The bias voltage on the squeegee-roller or absorbent roller produces an electric field which holds the toner firmly on the photoconductor surface. The bias voltage has the same polarity as the toner particles in the developer liquid; thus, the developed image remains adhered to the photoconductor surface without producing streaks or smears and without transfer of toner onto the squeegee-roller. After running past the squeegee-roller, the layer of liquid developer remaining on the photoconductor surface is reduced to a thickness of 2 to 3 .mu.m, so that, overall, the thickness of the layer of developer liquid on the photoconductor is reduced to approximately a fifth of the initial value.
The disclosure of Offenlegungsschrift No. 3,018,241 relating to copy quality is limited to the statement that no traces of dragging, streaks, or distortions should occur in the copy. Nothing is mentioned with regard to achievable copy density, which, particularly in the case of the squeegee-roller technique, is an important consideration since the squeegee-roller also squeezes off a proportion of the toner particles which have been deposited by electrophoresis on the photoconductor. Even if coarse streaks on the copies are avoided, very short streaks at the edges perpendicular to the direction of movement resulting from the squeezing off action adversely affect the edge-sharpness and, in consequence, reduce the resolution which can be achieved.
At the copy quality levels demanded at the present time, the resolution must be approximately six lines per mm, both in the direction of movement and at right angles to this direction, in order to be able to produce first-generation and second-generation copies which are easily readable, from the foregoing copies.
The liquid development technique offers advantages such as high resolution and low energy demand for fixing copies, as compared with the dry development technique. However, this technique also has the disadvantage that during the transfer of the toner image to the image-receiving material, the residual developer liquid which has not been squeezed off by the squeegee-roller is deposited on the image-receiving material and has to be evaporated by heating the copies during the fixing step. As a result, large quantities of developer liquid are lost and must be continually completed to the copier, and the air in the vicinity of the copier becomes undesirably laden with evaporated developer liquid. Although the customary developer liquids are not toxic per se, since in the majority of cases, they are aliphatic hydrocarbons such as i-decane in which the charged toner particles are dispersed, this large loss of developer liquid is undesirable because it leads to a certain level of environmental pollution.
In the present state of the art as described, for example in U.S. Pat. No. 3,907,423, the loss of developer liquid following the development of the charge-image on the photoconductive layer by electrophoretic deposition of charged toner particles is reduced by reducing the excess, projecting thickness of the layer of developer liquid. This reduction in thickness is carried out before the toner image is transferred to the image receiving material by a stripping roller rotating counter to the rotation of the photoconductor. The stripping roller rotates at a high peripheral speed counter to the movement of the photoconductive layer at a distance of only approximately 50 .mu.m from it. The toner-images deposited on the photoconductive layer are not smeared; however, only a portion of the projecting quantity of developer liquid is removed, so that moist copies are discharged.
Up until very recently, repeated attempts have been made to remove recess developer liquid from the photoconductor surface after the development of the electrostatic charge-image to effect a further reduction in the loss of developer liquid to the copies. In these attempts, both absorbent rollers made of a foamed polymer with open pores and squeegee-rollers have been employed. The squeegee-roller technique is described, for example, in U.S. Pat. No. 3,299,787. This patent discloses the use of a squeegee-roller with an associated cleaning element for removing the excess developer liquid from a photoconductive web.
In the present state of the art, there is a disadvantage in that when photoconductive drums coated with selenium are used, the toner-images developed on the photoconductor are smeared and/or distorted by freely-rotatable polyurethane squeegee-rollers used for removing excess developer liquid (German Offenlegungsschrift No. 3,018,241). Thus, although the squeegee-roller technique for reducing the loss of developer liquid has reached a defined, advanced state of development, it nevertheless continues to exhibit shortcomings which prevent it from being widely employed. With copying conditions according to the present state of the art, the successful production of copies with acceptable copy-densities of at least 0.7 and with a correspondingly good resolution of approximately six lines per mm, is not possible.